1. Field of the Invention
The present invention relates to a mapping device for producing mapping information about the disposition of plate-shaped objects, such as semiconductor wafers (hereinafter referred to simply as “wafers”).
2. Description of the Related Art
FIG. 10 is a front elevation of a transmission photoelectric sensor 1 included in a mapping device according to a first conventional technique. The mapping device produces mapping information about the disposition of wafers on the basis of detection information provided by a moving detecting unit, and position information about the position of the detecting unit. The transmission photoelectric sensor 1 is the detecting unit of the mapping device according to the first conventional technique.
The transmission photoelectric sensor 1 has a light projector 2 and a light receiver 3 spaced from the light projector 2. The light receiver 3 receives light projected by the light projector 2. A wafer 27 that passes a space between the light projector 2 and the light receiver 3 intercepts the light projected by the light projector 2 and hence the output of the light receiver 3 changes. The mapping device determines the position of the wafer 27 on the basis of the position of the detecting unit when the output of the light receiver 3 changes.
FIG. 11 shows a reflection photoelectric sensor 6 included in a mapping device according to a second conventional technique. The reflection photoelectric sensor 6 is the detecting unit of the mapping device according to the second conventional technique.
The reflection photoelectric sensor 6 has a reflecting mirror 7, and an optical sensing unit 8 disposed at a distance from the reflecting mirror 7 and including a light projector 9 and a light receiver 10. The light receiver 10 receives light projected by the light projector 8 and reflected by the reflecting mirror 7. The mapping device according to the second conventional technique, similarly to the mapping device according to the first conventional technique, determines the position of a wafer 27 on the basis of the position of the detecting unit when the output of the light receiver 10 changes.
As shown in FIG. 12, if the optical axis C1 of the light projector 2 (hereinafter referred to as “projection axis C1”) and the optical axis C2 of the light receiver 3 (hereinafter referred to as “reception axis C2) are not aligned in the transmission photoelectric sensor 1 according to the first conventional technique, the transmission photoelectric sensor 1 is unable to achieve correct detection. Positional adjustment of the light projector 2 and the light receiver 3 to bring the optical axes C1 and C2 into alignment needs troublesome work.
When determining the position of a very thin, plate-shaped object, such as the wafer 27, the light projected by the light projector 2 needs to be small. Faulty detection will results unless the optical axes C1 and C2 are aligned in a high accuracy when the light projector 2 projects light in a small-diameter light beam. Therefore, the respective positions of the light projector 2 and the light receiver 3 must be accurately adjusted. Such an accurate positional adjustment needs troublesome work.
As shown in FIG. 13, if the axis C5 of reflected light reflected by the reflecting mirror 7 (hereinafter referred to as “reflection axis C5”) is not aligned with the reception axis C4 of a light receiver 10 in the reflection photoelectric sensor 6 according to the second conventional technique, the transmission photoelectric sensor 6 is unable to achieve correct detection. Accurate positional adjustment of the reflecting mirror 7 and the optical sensing unit 8 to align the reflection axis C5 with the reception axis C4 needs troublesome work, similarly to the positional adjustment of the light projector 2 and the light receiver 3.